capi: Fix 'unused return value' warnings

[rhysd]

cargo check --workspace reported the following warnings on my local machine.

warning: unused return value of `alloc::ffi::c_str::CString::from_raw` that must be used
   --> regex-capi/src/rure.rs:261:17
    |
261 |                 CString::from_raw(ptr);
    |                 ^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(unused_must_use)]` on by default
    = note: call `drop(from_raw(ptr))` if you intend to drop the `CString`

warning: unused return value of `alloc::ffi::c_str::CString::from_raw` that must be used
   --> regex-capi/src/rure.rs:627:18
    |
627 |         unsafe { CString::from_raw(s); }
    |                  ^^^^^^^^^^^^^^^^^^^^^
    |
    = note: call `drop(from_raw(ptr))` if you intend to drop the `CString`

warning: `rure` (lib) generated 2 warnings

This PR fixes the warnings.

Environment

• rustc: 1.62.0
• cargo: 1.62.0
• OS: macOS 11.6.7

[rhysd]

Note: The #[must_use] changes were added at Rust 1.57: rust-lang/rust#89692

[BurntSushi]

We might as well go the whole nine yards here and add drop(..) around other calls to from_raw as well. e.g., Box::from_raw. Would you mind doing that as well?

[rhysd]

Thank you for the review comment. I searched Box::from_raw and applied the same fix at 1324f2d

[BurntSushi]

Perfect, thanks!